In producing flexible and semiflexible polyurethane foams having certain desired properties, it is often advantageous to employ a cross-linker. Many such cross-linkers have been used. The most widely used cross-linker in recent times has been 4,4'-methylene-bis-ortho-chloroaniline and mixtures containing such materials. Unfortunately, in pure form this material is a solid at convenient operating temperatures and thus is difficult to handle. The mixed materials containing this chlorinated aromatic amine cross-linker are also solids at room temperature and have been found to change with respect to their reactions in polyurethane compositions.
Recently methylene dianiline (MDA) has been tried as a replacement. While methylene dianiline gives many of the desirable properties of 4,4'-methylene-bis-orthochloroaniline, it is highly reactive, and additionally has a high melting point which makes it also difficult to handle when operating on a commercial scale. Prior art amine cross-linkers, including the above, are disclosed and discussed in U.S. Pat. Nos. 3,523,918 and 3,961,265, for example. However, these prior art cross-linkers have been deficient in some manner for use in polyurethane formulation, particularly for improving hardness of flexible foams. In addition, polyurethane foams using methylene dianiline cross-linkers have proved inferior to certain applications such as automobile crash pads, arm rests and packing materials because of their relatively low resistance to compressive set. Thus such foams are subject to excessive deformation upon extended use.
Cross-linkers of polymethylene polyphenylamine having a functionality of greater than 2.0 and being essentially 4,4'-isomer have also been used as cross-linkers. This cross-linker also proved hard to handle being extremely difficult to liquefy. Additionally, the freeze-thaw characteristics of this cross-linking material were found to be such that after several such cycles the performance in the polyurethane reaction varied appreciably. Further, the foams produced using these cross-linkers, while harder than comparable foams produced with MDA, still lacked suitable resistance to compressive set.
Surprisingly it has now been discovered that polymethylene polyphenylamine mixtures having an average functionality of from 2.2 to about 2.9 and having a methylene dianiline isomeric distribution of from about 10 to about 70 wt.% 2,4'-isomer, from about 1 to about 20 wt.% 2,2'-isomer, with the remaining methylene dianiline being 4,4'-isomer are extremely easy to handle and compatible with flexible or semi-flexible polyurethane foam formulations.
Additionally the above described mixtures have desirable freeze-thaw characteristics such that their performance in the polyurethane reaction does not vary even after several such cycles. Foams formulated using the polymethylene polyphenylamine mixture show improved hardness over foams formulated with MDA as well as superior resistance to deformation over foams formulated with a polymethylene polyphenlamine of substantially the 4,4'-isomer.